Enlarged amniotic cavity: a new sonographic sign of early embryonic death.

In the process of evaluating sonograms to determine the status of early gestations, it was noted that enlargement of the amniotic cavity appeared to correlate with embryonic death. This study tested that hypothesis by comparing the size of the amniotic cavity with the crown-rump length (CRL) and the size of the chorionic cavity in 25 normal gestations and 10 cases of embryonic death. Measurements included diameter of the amniotic cavity (Da), diameter of the chorionic cavity (Dc), and CRL. Normal first-trimester embryos have distinct rates of growth for both chorionic and amniotic cavities. Least-squares linear regression of CRL on Da for normal embryos reveals Da = 1.1 x CRL - 0.07 (r = .988, n = 25, p less than 10(-8)), indicating that Da and CRL are almost equal. An abnormal embryo, especially at 5-6 weeks' gestation, will have an abnormally large amniotic cavity for its CRL and the size of its chorionic cavity. The CRL-Da difference of 0.11 +/- 0.20 cm in normal embryos differed significantly from that difference of 0.86 +/- 0.38 cm in abnormal embryos (p less than 10(-8)); however, the chorionic cavity was still appropriate in size for the CRL. These results suggest that an amniotic cavity that is enlarged relative to the CRL and the size of the chorionic cavity is evidence of embryonic death.